Referring to FIGS. 1A-2C, there are shown schematic diagrams of operational steps, enlarged view and side views of operational steps of a conventional sealing machine. As illustrated, the sealing machine 20 of the prior art comprises two block members 22, two electrical heating wires 24 respectively mounted at the block members 22 opposite to each other, and two protecting belts 26 respectively covered on the electrical heating wires 24 to protect the electrical heating wires 24 against contamination of fused material of the plastic bag 10 during a sealing operation.
During operation of the aforesaid sealing machine 20, a plastic bag 10 that is filled with goods 15 are pressed together from the top and bottom sides or the opposite lateral sides around the opening by the block members 22, and the plastic bag 10 is folded along the folding lines 12. When the block members 22 are pressed on the plastic bag 10 against each other, the temperature of the electrical heating wires 24 causes the plastic material of the plastic bag 10 to be fused together and form a sealing line 14, as shown in FIGS. 1A-1C and FIGS. 2A-2C.
After the plastic bag 10 is folded, there are two four-layer film parts 16 and one double-layer film part 18 in the plastic bag 10 around the sealing position. The four-layer film parts 16 and the double-layer film part 18 require different sealing conditions (for example, the sealing temperature of the electrical heating wires and the pressure of the block members). The sealing conditions suitable for sealing the four-layer film parts may cause the double-layer film part to be excessively fused. At this time, under the effect of bilateral tension force of the plastic bag 10, a split 19 can be formed in the double-layer film part 18 of the plastic bag 10 around the sealing line 14, as shown in FIG. 1D.